In a computer tomograph (CT) the radiation used for medical imaging is generated by an X-ray tube. The spatial area of the anode of the X-ray tube, in which the X-ray radiation used for imaging is physically generated, can be assumed as being almost point-shaped in relation to the dimensions of the CT and the dimensions of a region of the body to be examined of a patient. The beam path of the X-ray radiation essentially fans radially from the generation point defined in this way, wherein the solid angle range, in which irradiation of the X-ray radiation occurs in the first place, can be influenced or restricted by the geometry of the anode and optionally a subsequent diaphragm arrangement.
The X-ray tube is arranged in the CT on a slewing ring which performs a rotational movement about an axis during operation of the CT. The region of the body of a patient to be imaged by the CT is positioned along this axis. The beam path of the X-ray tube is then guided in a fan shape, optionally with the aid of appropriate diaphragms, from the generation point into the interior space surrounded by the slewing ring.
For optimally high image resolution by the CT, an optimally sharp contrast is desirable in each case in every single image recording, which is produced by an X-ray beam generated in a particular angular position or in a narrow angular range of the generation point. Since the contrast in an individual scan is produced by the different absorption level for X-ray radiation, which the individual tissue layers that are to be imaged have, and since, further, the tissue to be imaged in each case during a scan is precisely defined, firstly an optimally high intensity of the incident X-ray radiation is desirable for an increase in the contrast and therewith an improvement in the noise. Secondly, for medical reasons this radiation intensity should be limited, however, at least averaged over time, for a specified region of the body.
In order to limit the intensity of the X-ray radiation a CT often has a number of absorption filters which are arranged in the vicinity of the generation point, and therefore limit the intensity striking the patient from the start. A constant absorption profile achieved hereby and the accompanying predefined limitation of the radiation intensity is not usually capable of adequately taking into account anatomical peculiarities of the region of the body to be examined, however. This should be considered, in particular, against the background that the X-ray tube rotates around the region of the body of the patient to be imaged during operation of the CT.
An anatomical peculiarity can therefore result, for example, from an eccentric position of the region of the body in respect of the axis of rotation, but also from the fact that radiation striking the front of the patient covers an essentially shorter distance through the patient, and consequently undergoes significantly less absorption, than radiation that strikes the side of the patient, which propagates, for example, from one shoulder to the opposite shoulder. The difference in the level of absorption solely due to the different distances covered in the body of the patient can amount to more than a factor of 100. Previous absorption filters do not provide a satisfactory solution to this. Furthermore, a variation in the spatial absorption profile, in particular during a CT scan, desired for the reasons above is currently neither provided nor feasible.